This invention relates to an ionization chamber assembly for use with a smoke detector and, more particularly, to such an assembly having both an open sensing chamber and a closed reference chamber.
Double chamber ionization chamber assemblies are well known. One such type of assembly has a pair of cup-shaped electrodes respectively defining the outer walls of an open sensing chamber and a closed reference chamber. An interior wall is interposed between the two cup-shaped electrodes and defines an interior wall of both chambers. The wall also carries a centrally located electrode common to both chambers. Smoke detection circuits employing such assemblies apply a voltage across the assembly between the two cup-shaped electrodes and monitor the voltage developed at the common electrode. Thus, voltage varies as an impedance voltage divider output when smoke or other products of combustion enter the sensing chamber and increase its impedance relative to that of the closed reference chamber.
A problem associated with this type of chamber assembly and others which have an electrode contained within a conductive envelope is the provision of an inexpensive means to connect external circuitry with the center electrode without making electrical contact with the conductive envelope. A functionally successful, but relatively expensive, approach to this problem, shown in U.S. Pat. No. 3,832,552 of Larsen et al issued Aug. 27, 1974, to Honeywell, Inc., is to mold the insulator wall with the metal conductor protectively embedded therewithin and thereby insulated against connection with the cup-shaped electrodes.
The impedance of the ionization chambers is extremely high, and thus the current therethrough giving rise to the detection signal is very low, being in the range of a few pico-amperes. Accordingly, it is extremely important that the circuit element connected with the center electrode and used to detect these currents be virtually devoid of leakage currents between the various terminals thereof and between individual ones of the terminals and the different parts of the ionization chamber assembly. Typically, field effect transistors, or the like, are used as this circuit element because of their high input impedance and their inherently low leakage current characteristics. However, even with such devices, moisture, dust and the like, particularly at the header surface of the device from which the leads protrude, can result in the development of leakage currents having sufficiently large magnitudes relative to the very low ionization chamber current that proper response to the ionization current is prevented.
The several known approaches which have been taken to minimize such leakage current have also significantly increased manufacturing cost. For example, in U.S. Pat. No. 3,681,603 of Scheidweiler et al, issued Aug. 1, 1972, to Cerberus A. G., a separate cavity in a mounting member of the detector is provided for protectively enclosing the sensing circuit. In U.S. Pat. No. 3,710,110 of Lampart et al, issued Jan. 9, 1973, to Cerberus A. G., the detection circuitry is mounted between two center electrodes and protectively encapsulated in a moisture-impervious insulating material. A structurally complicated ionization chamber assembly having a reference chamber mounted within a sensing chamber is disclosed in U.S. Pat. No. 3,500,368 of Abe, issued Mar. 10, 1970, to Nittan Company, Limited, in which the body of a field-effect transistor is mounted in a hole of an insulator member defining a wall of the reference chamber. Another known approach to protect the header has been to coat it with a moisture-impervious enamel.